You have an heated Oxygen sensor. 2 of the wires heat up the sensor and keep it at a certain temperature, it is turned on and off by the computer. The other 2 are use to send a very low voltage reading back to the computer to tell the computer if the engine is running rich or lean. I don't have a diagram to tell you which wire does what but you can get a manual and that should give you the information that you need and tell you how to trouble shoot it to tell if the sensor is bad or not, you may want to look for a wire laying on the exhaust, I have found wires burnt and lay on the metal.

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The sulfur smell is because the 4 cyl is running rich . The issue can be the maf sensor or the 02 sensor going bad or the car needing a tune up. If the sensors are going bad there should be a check engine light on. Read the code for free at any parts store and then google the code they give you for more info on fixing the problem.

Since you said 4 sensors I will assume you have the V6 engine.The aftermarket sensors that are designed to fit your truck are about $150 each so with labor that would be about right. And sensors from GM would probably be $250 each. However, odds are you do not need all 4 sensors. Maybe one.

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms: You will likely not notice any drivability problems, although there may be symptoms.

Causes: A code P0420 may mean that one or more of the following has happened: Leaded fuel was used where unleaded was called for An oxygen sensor is not reading (functioning) properly The engine coolant temperature sensor is not working properly Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe Retarded spark timing The oxygen sensors in front and behind the converter are reporting too similar of readings

Some suggested steps for troubleshooting a P0420 error code include: Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required. Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady). Inspect the downstream heated oxygen sensor (HO2), replace if necessary Replace the catalytic converter

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

It has to be an oil pressure sending unit. There are two of them on that SUV but they are in different places depending on your engine cord. If you go to Advance Auto parts or O'Riley, they will tell you exactly where to look.